The present invention pertains to electron microscopes. Specifically, this invention pertains to a sample mount for use with a scanning electron microscope.
The semiconductor wafer fabrication process relies heavily on physical inspection processes to ensure product quality. Due to the minute size of the wafer features, highly specialized equipment is required. This equipment typically includes a variety of viewing instruments such as microscopes which permit a technician to accurately magnify and view specific features of the wafer sample.
For various reasons, conventional optical microscopes are ill-fitted for wafer inspection. For example, they are limited in their ability to resolve detail at a level sufficient to enable adequate wafer examination. Additionally, they are unable to achieve the required magnification levels. Furthermore, depth-of-field (i.e., the ability to keep objects at two different depths simultaneously in focus) is restricted, requiring the operator to constantly re-focus the microscope as different areas of the sample are inspected.
These drawbacks are eliminated by using a scanning electron microscope (SEM). Unlike an optical microscope, the SEM utilizes an electron beam to bombard the sample as it sits within a vacuum environment. Due to the characteristics of the electron beam (as opposed to the visible light source used in optical microscopes), resolution and magnification are significantly increased. Additionally, no depth-of-field problems exist with the SEM so surfaces at any depth can be examined without re-focusing. These advantages have made the SEM essential to the wafer inspection process.
Before viewing the wafer sample in the SEM, the wafer must be securely mounted. Typically, the microscope includes a movable base to facilitate specimen mounting. However, an appropriate sample mount is necessary to secure the sample to the base. The sample mount used varies depending on the wafer features to be inspected. For example, sample mounts are known for inspecting the face of the wafer while other mounts permit inspection of wafer edge features. The present invention is addressed to the latter and the remainder of this discussion is directed accordingly.
One apparatus is described by the Applicant herein in a co-pending, commonly assigned application entitled xe2x80x9cWafer Sample Retainer for an Electron Microscope,xe2x80x9d filed on Dec. 01, 1997 having Ser. No. 08/980,932.
For semiconductor wafers, inspection of edge features is usually accomplished by securing several wafers together and mounting the sample in a vertical orientation relative to the SEM. The mount typically consists of a vertical member to which one or more wafer specimens are secured using a curable adhesive. Copper tape is then wrapped around the specimens and the mount to secure the sample. While such mounts have proven effective, drawbacks exist. For example, the application and removal of the tape adds additional steps to the inspection process. Additionally, the curable adhesive may require several hours to cure prior to inspection. Furthermore, periodic cleaning of the fixture may be required to remove adhesive residue.
Thus, there are issues concerning increased setup time with current semiconductor sample retaining devices. As wafer fabrication facilities continue to increase production rates, the total number of wafers inspected must also increase. As a result, there is a need for a sample mount that provides quick and effective mounting without the drawbacks inherent with adhesives.
A sample mount for an scanning electron microscope (SEM) is disclosed in which the mount comprises a first clamp member, a second opposing clamp member, and a lead screw operatively connected to both clamp members. Rotation of the lead screw varies the distance between the clamp members. A method for retaining a sample for examination in a SEM is also disclosed comprising securing a sample mount to a base, inserting a sample into the sample mount, and turning a lead screw in a first direction to move a first clamp member toward a second clamp member, thereby securing the sample therebetween.
The sample may be a single silicon wafer or a plurality of wafers. Various sample thicknesses may be accommodated by merely turning the lead screw to move the clamp members relative to one another.
The sample mount may be removably mounted to a base on the SEM. The SEM may further include a rail in which the base is adjustably positionable. The base may be positionable with a motor-driven screw.
In one embodiment, both clamp members are movable relative to the base. In another embodiment, one clamp member is fixed relative to the base and the other clamp member moves relative thereto.
The sample mount may comprise a retaining assembly having a plate and a clamp body removable attached to the plate. A first and second clamp member may be operatively connected to the retaining assembly whereby the clamp members are capable of securing a sample therebetween. A lead screw may be secured to the retaining assembly and operatively connected to the first and second clamp members, whereby rotation of the lead screw varies the distance between the clamp members. A thumb-wheel may be provided at one end of the lead screw to assist the operator in turning the screw. The retaining assembly, first and second clamp members, and the lead screw may all be removably secured to a base on the SEM.
In one embodiment, the lead screw may comprise a central threaded portion having a first threaded portion and a second threaded portion wherein the first threaded portion has a right-handed thread and the second threaded portion has a left-handed thread. The first clamp member is threadably engaged to the first threaded portion of the lead screw and the second clamp member is threadably engaged to the second threaded portion such that rotation of the lead screw in a first direction results in relative closure of the clamp members. Alternatively, rotation of the lead screw in a second direction results in relative separation of the clamp members. Regardless, the clamp members move relative to a common origin.
The present invention provides an improved sample mount that permits quick and efficient edge mounting of wafer samples within a SEM. Furthermore, mounting is accomplished without the use of messy adhesives and tapes. By avoiding the use of adhesives, the sample mount does not require the lengthy cure time often associated with adhesive mounts. Advantageously, inspection throughput is increased, preventing wafer inspection from becoming a production bottleneck.